It is impossible for us to teach even the basics of English grammar in one chapter. Instead, we aim to drive home the point that it is important. Reading something with bad grammar is unpleasant, and will drive readers away.

Decent grammar will work in your favor.

A few, key, commonly made mistakes:

1- Know the difference in use within the following groups of words:

(a) Their/They're/There
(b) Your/You're
(c) To/Too/Two
(d) Its/It's
(e) Affect/Effect (this is my personal demon -a)
*Very general rule of thumb - if discussing the noun, use effect: The drug I took had great effect. If using as a verb, use affect: I was affected by the drug.
(f) Peak/Peek/Pique
(g) Then/Than
(h) Assure/Ensure/Insure
(i) Complement/Compliment
(j) Accepted/Excepted
(k) Decent/Descent
(l) Dominant/Dominate
(m) Bare/Bear

(n) Definitely/Defiantly

(o) Rogue/Rouge

2- If you use 'a' as a word before another word starting with a vowel, it becomes 'an.'
e.g., A major incident vs. She thought an incident was major

3- The first letter of the first word in a sentence is capitalized.

(a) unless you're making a conscious stylistic choice. though fine for chatting, it's harder to read. -a

4- Use punctuation correctly. If you don't know how to do this, there are resources online regarding how.

5- Within a paragraph, stick with a consistent perspective and tense.

6- Use the break of a line (spaces) between paragraphs. One lengthy block of text is an eyesore.

7- Pick a point of view and stick with it for a chapter. (Third person can switch perspectives mid-chapter more seamlessly, but you should make whose perspective you are changing to clear immediately.) But don't switch mid-paragraph, whatever you do.

(a) First person, past tense: I wanted to eat the apple. First person, present: I want to eat the apple.

(b) Second person, past tense: You wanted to eat the apple. Second person, present tense: You want to eat the apple.
*note- Second person is very uncommonly written in. See the Fighting Fantasy books, or other Choose Your Own Adventure books.

(c) Third person, past tense: Helga wanted to eat the apple. Third person, present tense: Helga wants to eat the apple.

8- An advanced tip: try not to use too much passive voice. Passive voice is when you put the object of a sentence at the beginning of a sentence, and have the verb following. It makes your sentences sound less clear:

(a) I was looking for the apple. (active voice = stronger)

(b) The apple was what I was looking for. (passive voice)

9- Possessive nouns (i.e., where do you put the apostrophe?):
(a) With a plural noun, after the s: The ladies' room.
(b) With a singular noun ending in s, after the s: The lass' dress
(c) With a singular noun not ending in s, apostrophe, add an s: The lady's dress

10- "Alot" is not a word. Use "A lot," unless you mean "Allot."

11- Into = an indicator of movement: He strode into a room.
In to = anything else: I called in to the office to let them know I'd be late.

12- Lose vs. Loose:
Lose = to lose, as in a game or argument: The Bengals are going to lose often, you get used to it as a fan.
Loose = not tight or to let fly: Her hair was loose about her shoulders.; I let loose a mighty fart.

13- The semicolon: Think of it as punctuation to separate two statements that could otherwise stand on their own; this is an example sentence.

14- Have subject-verb agreement.
ex. Good = I have three ponies. Bad = I has three ponies.

15- Another advanced tip: Don't use dangling modifiers. This means, have your modifier (adjective or adverb) preceding what it modifies, not after it.
ex. Good = I looked at the expensive men's watch. Bad = I looked at the men's expensive watch.

16- Breath = a noun. Breathe = a verb: I took a breath vs. I took a moment to breathe.

17- Beware the run-on sentence.

Notes

Note- I am terrible about the passive voice thing, myself. You?

this was way more thorough than i thought it would be. you make me feel like such a shitty writer, haha! -a

1-(n) & 1-(o) thanks to  Starelf555